1. Field of the Invention
The present invention is related to removing reaction byproducts from reaction effluents in thin film deposition processes and apparatus, and, more specifically, to a method and apparatus for preventing titanium tetrachloride and solid products of titanium tetrachloride reactions with ammonia from forelines of reaction chambers, for example, those in which titanium tetrachloride and ammonia feed gases are reacted to deposit titanium nitride.
2. State of the Prior Art
A common problem in vacuum deposition system, such as low pressure chemical vapor deposition (LPCVD) and atomic layer deposition (ALD) systems, is handling the effluents flowing out of the reaction chambers, including trying to prevent or at least minimize excessive wear in vacuum pumps caused by solid products or byproducts of reactions of feed gases. These problems are often unique to the particular feed gases, reactions, and conditions used in the processes, so solutions to such problems in one deposition system may not be as effective, or even effective at all, in other systems.
One particularly vexing problem has been excessive vacuum pump wear in titanium nitride deposition system, where vacuum pumps typically have useful lives of less than thirty (30) days and often as little as two or three days. Installation of condensation traps, such as that described in U.S. Pat. No. 5,820,641, in the foreline of LPCVD TiN deposition systems have been effective to condense and trap byproducts before they reach and damage the vacuum pumps in some of the older LPCVD TiN deposition systems, but they do not solve the problem in some of the more recent LPCVD TiN deposition systems and practices, as will be discussed in more detail below.
Titanium nitride (TiN) is a very dense material that has a number of uses in thin film form, such as for separating copper or aluminum conductor layers in semiconductor devices from silicon dioxide layers or substrates so that the copper or aluminum ions cannot react with, or diffuse into, the silicon dioxide material. A common method of producing thin films of titanium nitride (TiN) on substrates is to react titanium tetrachloride (TiCl4) with ammonia (NH3) in a low pressure, chemical vapor deposition (LPCVD) process chamber. The chemical reactions inside the process chamber (sometimes also called the reaction chamber) include the following:
Deposition of TiN:6TiCl4+8NH3→6TiN+N2+24HCl  (1)
Formation of ammonium chloride:NH3+HCl→NH4Cl  (2)
Dissociation of TiCl4:TiCl4→TiCl3+Cl  (3)
Adduct formation in the gas phase:TiCl4+2NH3in a cold wall reactor>TiCl4.2NH3 (yellow solid)  (4)
The target reaction for the deposition of TiN on a substrate in the process chamber is, of course, equation (1) above. The other reactions (2), (3), and (4) are superfluous for the deposition of TiN, but they cannot be avoided. The ammonium chloride (NH4Cl) produced according to equation (2) and the TiCl4.2NH3 adduct produced according to equation (4) are by-products which are condensable to solid materials, and, when they condense to solid materials in the foreline and vacuum pump components, they cause the abnormal wear and damage to vacuum pumps discussed above. The ratio of the desirable TiN to the undesirable TiCl4.2NH3 produced in the process is very small, so there are large amounts of TiCl4.2NH3 produced. Fortunately, however, since TiCl4.2NH3 condenses readily to a yellow solid, a water-cooled trap in the foreline has been quite effective for trapping and removing substantially all of the TiCl4.2NH3 adduct from the process chamber effluent before it could reach and damage the vacuum pump in conventional LPCVD TiN deposition systems where the reactant feed gases are turned on only during deposition and are turned off when the coated products are being removed from the chamber.
However, a more recent practice of diverting vaporized TiCl4 around the process chamber and dumping it directly into the foreline or directly into the water cooled trap while the TiN coated wafer substrates are removed from the process chamber and replaced with new, uncoated wafer substrates has been found to produce a different byproduct in the form of a yellow powder in addition to the usual yellow solid, and the conventional water-cooled condensation traps used to prevent the solid TiCl4.2NH3 adduct from damaging the vacuum pump are ineffective at preventing the yellow powder byproduct from reaching and damaging the pump. It has also been observed that the conventional water-cooled condensation traps do not prevent solid TiCl4.2NH3 from also damaging the vacuum pump when TiCL4 is diverted around the process chamber as described above. This TiCl4 diversion and dumping practice is becoming more common, because TiCl4 is normally liquid at room temperature and has to be vaporized for feeding into the process chamber. Vaporizers cannot be turned on and off instantaneously, so, in this more recent practice, they are left on to operate at a steady state, and the flow of TiCl4 vapor is simply diverted around the process chamber by a three-way valve or other appropriate valve mechanism when the wafers are being changed. Then, when the new wafers are in position and the process chamber is closed and ready for deposition, the flow of TiCl4 is switched back into the process chamber to react with the NH3. The practice enables almost instantaneous on/off control of the TiCl4 vapor flow into the process chamber, but it adds substantial amounts of unreacted TiCl4 downstream of the process chamber, and the vacuum pump life has become extremely short when this practice is used.
Similar problems are encountered in atomic layer deposition (ALD) processes in which the reactant respective feed gas flows into the process chambers are switched alternately on and off as depositions are built one atomic layer at a time. For example, atomic layer deposition of TiN involves alternating sequential, pulsed flows of TiCl4, a purge with N2, then a pulse of NH3, followed by another purge with N2, and starting over again with the TiCl4. These sequential pulses of reactant gases TiCl4 and NH3 in a high temperature, low pressure reaction chamber result in atomic layer deposition of TiN on a wafer substrate in the process chamber, but portions of the feed gas that do not react in the process chamber mix with each other and react downstream in the foreline and vacuum pump to cause the same excessive wear problems in the vacuum pump as described above.